The invention relates to a self-drilling and thread-forming screw according to the preamble of claim 1.
G 92 11 354.0 discloses such a self-drilling and thread-forming screw. To form a cylindrical passage in a metal sheet with such a screw, however, requires a high expenditure of force with the screw, consisting of a drilling section, a flow-forming section and a threaded shank, since there is no uniform transition, but rather an edge, between the essentially cylindrical drilling section and the essentially conically running flow-forming section, which is also designated as a flaring section. In the region of the chip groove extending from the cutting edges of the drilling section, a shoulder is formed between the cylindrical drilling section and the conical flaring section. This shoulder results from the fact that the chip groove at the end of the cylindrical drilling section runs out virtually at right angles to the screw axis and a material accumulation occurs above the chip groove, which material accumulation has a distinctly greater diameter than the effective diameter, formed by the cutting edges, of the drilling section. On the one hand, this shoulder requires a substantially larger contact pressure to be exerted on the screw after the relatively easy penetration by the drilling section in order to press the described shoulder through the sheet and to allow the conical flaring section to come into effect; on the other hand, such a shoulder greatly reduces the applicability for higher-strength sheets. Since in this known screw the chip groove merely extends over the length of the drilling section, the drill chips which collect in this chip groove and expediently have to be discharged may jam between the sheet material and the shoulder described above, so that the friction heat which is generated is transmitted essentially to the chips and not to the sheet. This impairs the flow-forming operation inasmuch as the flow-forming section may tend to form cracks during the forming.
U.S. Pat. No. 5,141,376 discloses a self-drilling and thread-forming screw which has a drilling section, a conically running region provided with thread turns, and a threaded shank. Directly after the drilling of the sheet with the drilling section, the thread is formed by the conical threaded region directly adjoining the drilling section, which corresponds to the forming of a thread in a plurality of production stages. Such a self-drilling screw has the disadvantage that, when screwing into thin sheet, the thread bites immediately after the sheet is drilled through and the drilled hole may tear due to a forced feed occurring after the biting of the thread. Therefore only an inadequate thread depth and thus inadequate retaining stability are achieved with such a self-drilling screw, which leads to a limited overtorque. Furthermore, cracking of the sheet opened out with such a screw cannot be ruled out. In addition, the drilling region directly adjoins the conical threaded section so that the chip grooves located in the drilling section end at the region in which the conically formed thread begins. The chips produced in the drilling section by the drilling operation are therefore conveyed directly into the region in which the thread is formed. As a result, not only is considerable friction energy likewise passed into the chips instead of into the hole region to be opened out but the chip material which gets in the way during the formation of the thread may lead to incorrect formation of the thread and thus likewise to a reduction in the overtorque.
A beading and thread-cutting screw is described in DE-AS 22 43 707. This beading and thread-cutting screw has a cylindrical threaded shank and a flaring section tapering towards the screw end and having a non-circular cross-sectional area, the non-circular cross-sectional area preferably being designed as an elliptical area. The flaring section is designed in such a way that it has smooth peripheral regions provided with thread-turn sections. The fact that the axis of the conical flaring section is offset from the axis of the threaded shank and a guide pin which is coaxial with the threaded shank adjoins the flaring section is intended to ensure that the eccentric flaring section beads the margin of a hole. Although in one exemplary embodiment the lower region of this screw has a chip groove which extends over the drilling section into the flaring section, the drilling section merges into the flaring section without a smooth transition due to a sharp edge, and furthermore the chip groove is arranged in front of the chip edges in the direction of rotation so that the chips removed from the chip edges collect in the chip groove during the drilling operation and cannot be readily discharged from the chip groove. There is therefore the risk that, during the beading operation following the drilling, the chips will primarily absorb the friction energy produced, as a result of which the flow properties of the material deteriorate and cracking may occur. In addition, a relatively large contact force of the screw is required to overcome the discontinuous transition between the drilling section and the flaring section in order to allow the flaring section to come into effect. In such a screw, the flow operation is also associated with considerable vibrations, which are transmitted in an intolerable manner either to the workpiece to be fastened or via the screw to the screwdriving tool. Since the drilling section designated as guide pin can no longer serve as guide pin after passing through the sheet, uniform forming or furrowing of the thread is not possible, since thread turns formed first can destroy following thread turns. In addition, such a self-drilling and thread-forming screw is not possible in particular for higher-strength sheets, the use of which is gaining in importance.
U.S. Pat. No. 3,937,120 discloses a thread-forming screw which has a leading or guide drilling tip from which two longitudinal chip grooves extend which have generally flat and essentially coplanar radial walls which form the side cutting edges. The front region of the thread-forming screw has a drilling section as well as a conical area extending up to the threaded shank, in which case the cutting edges running from the drill tip extend in the axial direction due to formed chip grooves up to the start of the threaded shank but run discontinuously due to discontinuously adjoining sections. In such a screw, material in the form of chips is therefore cut out of the hole up to the region of the threaded shank and can thereby be carried directly into the region in which the thread is to be formed. The forming of the thread is therefore permanently disturbed by the presence of chips, for which reason the overtorque which can be achieved with such a screw is very limited due to an incorrectly formed thread.